How to define an empty object in PHP

I want to point out that in PHP there is no such thing like empty object in sense:

$obj = new stdClass();
var_dump(empty($obj)); // bool(false)

but of course $obj will be empty.

On other hand empty array mean empty in both cases

$arr = array();
var_dump(empty($arr));

Quote from changelog function empty

Objects with no properties are no longer considered empty.

The standard way to create an "empty" object is:

$oVal = new stdClass();

But I personally prefer to use:

$oVal = (object)[];

It's shorter and I personally consider it clearer because stdClass could be misleading to novice programmers (i.e. "Hey, I want an object, not a class!"...).

---

(object)[] is equivalent to new stdClass().

See the PHP manual (here):

stdClass: Created by typecasting to object.

and here:

If an object is converted to an object, it is not modified. If a value of any other type is converted to an object, a new instance of the stdClass built-in class is created.

and here (starting with PHP 7.3.0, var_export() exports an object casting an array with (object)):

Now exports stdClass objects as an array cast to an object ((object) array( ... )), rather than using the nonexistent method stdClass::__setState(). The practical effect is that now stdClass is exportable, and the resulting code will even work on earlier versions of PHP.

---

However remember that empty($oVal) returns false, as @PaulP said:

Objects with no properties are no longer considered empty.

Regarding your example, if you write:

$oVal = new stdClass();
$oVal->key1->var1 = "something"; // this creates a warning with PHP < 8
                                 // and a fatal error with PHP >=8
$oVal->key1->var2 = "something else";

PHP < 8 creates the following Warning, implicitly creating the property key1 (an object itself)

Warning: Creating default object from empty value

PHP >= 8 creates the following Error:

Fatal error: Uncaught Error: Undefined constant "key1"

In my opinion your best option is:

$oVal = (object)[
  'key1' => (object)[
    'var1' => "something",
    'var2' => "something else",
  ],
];

$x = new stdClass();

A comment in the manual sums it up best:

stdClass is the default PHP object. stdClass has no properties, methods or parent. It does not support magic methods, and implements no interfaces.

When you cast a scalar or array as Object, you get an instance of stdClass. You can use stdClass whenever you need a generic object instance.

Short answer

$myObj = new stdClass();

// OR 

$myObj = (object) [
    "foo" => "Foo value",
    "bar" => "Bar value"
];

---

Long answer

I love how easy is to create objects of anonymous type in JavaScript:

//JavaScript
var myObj = {
    foo: "Foo value",
    bar: "Bar value"
};
console.log(myObj.foo); //Output: Foo value

So I always try to write this kind of objects in PHP like javascript does:

//PHP >= 5.4
$myObj = (object) [
    "foo" => "Foo value",
    "bar" => "Bar value"
];

//PHP < 5.4
$myObj = (object) array(
    "foo" => "Foo value",
    "bar" => "Bar value"
);

echo $myObj->foo; //Output: Foo value

But as this is basically an array you can't do things like assign anonymous functions to a property like js does:

//JavaScript
var myObj = {
    foo: "Foo value",
    bar: function(greeting) {
        return greeting + " bar";
    }
};
console.log(myObj.bar("Hello")); //Output: Hello bar

//PHP >= 5.4
$myObj = (object) [
    "foo" => "Foo value",
    "bar" => function($greeting) {
        return $greeting . " bar";
    }
];
var_dump($myObj->bar("Hello")); //Throw 'undefined function' error
var_dump($myObj->bar); //Output: "object(Closure)"

Well, you can do it, but IMO isn't practical / clean:

$barFunc = $myObj->bar;
echo $barFunc("Hello"); //Output: Hello bar

Also, using this synthax you can find some funny surprises, but works fine for most cases.